30 



The relative ammonifying efficiency of the soil at this time was 

 determined to be only 2.48 and the total acidifying efficiency 

 1 .81. The bacterial or zymotic efficiency of this soil is therefore 

 much lower than it was at the first examination. 



On May II, 1903, a sample of soil from sandy land which had 

 been brought into a good state of fertility was examined, with 

 the following results : 



Per gram 

 of dry soil. 



Bacillus ellenbachiensis . . 140,000 



Bacillus alcaligenes var delavariensis . 380,000 



Bacillus tumescens . . 20,000 



Total . . 540,000 



The relative ammonifying deficiency is 8.9 ; the acidifying effi- 

 ciency only 0. 58. It is interesting to note that the bacterial effi- 

 ciency of this soil is nearly three times as great as that of the sta- 

 tion soil at the second examination, though the latter contained 

 approximately three times the number of bacteria. This is ex- 

 plained by the great efficiency of Bacillus ellenbachiensis. 



Nitrification and nitrogen-fixing activity can be determined 

 and expressed on a relative basis in a similar manner. The ad- 

 dition of available nitrogen to soils through the decay of nitro- 

 genous matter can never exceed or even approximate the amount 

 taken from the soil in the production of that organic material. It 

 represents a gain in nitrogen only in the sense that it is saved. It 

 is not desirable that its conversion into soluble form should much 

 exceed the demand of the crop; otherwise it may be lost. It is 

 evident, however, that with the great waste of organic matter 

 which must inevitably go on we must have other sources of nitro- 

 gen to cover the loss and meet the rapidly increasing demand for 

 it, not only in agriculture but in other arts. 



THE FIXATION OF ATMOSPHERIC NITROGEN BY SOIL BACTERIA. 



Leaving, now, the question of the changes in the nitrogenous 

 organic matter in the soil, we will consider the fixation of atmos- 

 pheric nitrogen by a class of bacteria that uses the carbohydrate 

 constituents (sugar, starches, cellulose, &c.) of the vegetable mat- 

 ter in the soil. There are two classes of bacteria that can fix 

 atmospheric nitrogen : (l) Those that are not associated with any 

 particular crops, and (2) the root-nodule forms associated princi- 

 pally with legumes. The first group depends on the carbohydrate 

 material in the organic matter of the soil derived from decaying 

 vegetation or from certain minute algae (the Cyanophyceae or 

 blue-green algae). The second group depends principally on the 

 carbohydrates supplied by the plants in the roots of which the 

 bacteria are growing. The latter class will be considered later. 



The independent soil forms are widely distributed and belong 

 to several genera. A very good comparison of some of these has 



